This is an interdisciplinary research project between the departments of neuroradiology, neurosurgery and head and neck surgery. During the last two years we have developed several extremely low viscosity silicone rubber formulations suitable for the embolization of vascular tumors and malformations. In 1974, we developed a 60 centistoke (cstk) silicone rubber formulation and used it extensively in patients. At that time this represented a tenfold decrease in viscosity of the silicone formulations suitable for this purpose. In 1976 we lowered the viscosity further to 4 cstk. In 1978 we developed a 4 cstk formulation which was capable of producing an expanding silicone foam upon polymerization. These materials have been tested for toxicity in animals, particularly to determine their effect on the vascular wall. Both the scanning electron microscope and the light microscope were used for evaluation of the results. Our intention is to use these extremely low viscosity materials in patients to achieve a more effective and safer embolization procedure which can be made possible by the use of very small catheters. We propose to use the foam silicone rubber clinically after completion of the animal experimentation now in progress. We also plan to develop in the laboratory a technique to effectively include cytotoxic material in the silicone rubber and study its diffusion characteristics. The ultimate purpose of this aspect of the project is to determine whether these substances will eventually be useful for the management of neoplasms.